Since the standards relating to mobile telecommunications systems are different throughout the world, but also because an ever increasing number of different standards is being offered within a transmission area depending on the required bandwidth, it would be desirable to have a mobile telecommunications device capable of covering all these standards or at least some of them. In this situation, the problem consists in the fact that the send and receive bands lie in different frequencies and moreover have different bandwidths. An electronically tunable filter for telecommunications devices is therefore desirable which enables the telephone to be set to the desired standard by means of software.
Up to now there have been no tunable filters which solve the above problem. A workaround for this is offered by using the surface acoustic wave filters (SAW filters) which are proven in telecommunications technology and switching back and forth between these filters as required. Surface acoustic wave components are used in many variations as wirelessly interrogable or self-sending sensors, identification markers or filters. Embodiments according to the prior art are published for example in DE 198 60 058 C1, EP 0 746 775 B1, EP 0 655 701 B1, EP 0 651 344 B1, EP 0 619 906 B1, U.S. Pat. No. 5,691,698, U.S. Pat. No. 5,841,214, U.S. Pat. No. 5,966,008,U.S. Pat. No. 5,910,779, U.S. Pat. No. 6,029,234, U.S. Pat. No. 6,084,503.
SAW filters themselves are known for their stability and can therefore unfortunately not be tuned or can only be tuned within very tight limits. The use of a plurality of filters in one device naturally results in a greater space requirement and increased costs since a separate filter is required for each standard. Since, in addition, these SAW filters are not capable of being integrated into semiconductor technology, hybrid solutions must be sought.
In order to at least get away from these hybrid solutions, FBARs (film bulk acoustic resonators), which operate using bulk acoustic waves are frequently proposed as they can, at least in part, be manufactured to be CMOS compatible. However, a separate filter or even a plurality of filters is also required in this situation for each standard. The main problem with this solution, however, consists in the fact that the required frequency precision cannot be achieved on account of technical manufacturing considerations, with the result that mass production and accompanying low prices are not available.
An FBAR is known from EP 0 834 989 A2 whose elastic constants are modifiable at least in part. However, these changes are so small that they do not allow variations in production to be brought under control, let alone allow a tunable filter to be implemented.
R. E. Newnham, A. Amin: “Smart Systems: Microphones, fish Farming, and beyond” CHEMTECH, [Online] Vol. 29, No. 12, 1 Dec. 1999, pp. 38 to 46, American Chemical Society, discloses piezoelectric ceramics having compounds close to the second phase transition. This means that load is measured in sensors and load is generated in actuators.
U.S. Pat. No. 4,464,639 describes the properties of ferroelectric crystals, in particular the relationship between ferroelectric state and piezoelectric state in such crystals and the ability to modify pressure and load by means of electric fields. Furthermore, the phase transition from the ferroelectric phase to the paraelectric phase at the Curie point is described. A method is described for setting the frequency properties of a crystal by means of polarization above the transition temperature and operation beneath the transition temperature, whereby twice the frequency of the interdigital converter can be attained. Finally, an application is illustrated for a filter which is tuned above the transition temperature and operated beneath the transition temperature.